Studies on the Zeolites. Part VIII. A Theory of the Vapour Pressure of the Zeolites, and of the Diffusion of Water or Gases in a Zeolite Crystal1

Max H. Hey
Assistant-Keeper in the Mineral Department of the British Museum of Natural History
1Parts I–VII, Min. Mag., 1930–1934, vols. 22–23.

Summary: A simple kinetic treatment leads to an equation for the dissociation pressure of a ‘zeolitic’ compound of a volatile and a non-volatile component: logep=loge4NR(ε−η)∕aNDXπ+12logeT−logex∕(1−x)−N(ε−η)∕RT, where p is the equilibrium pressure in dynes per sq. cm., N the Avogadro number, R the gas constant in ergs, T the absolute temperature, ε and η the activation energies in ergs per molecule of volatile component for dissociation and re-combination respectively, x the fraction of unoccupied lattice positions for the volatile component (assumed all of the same kind), D the distance between lattice positions for the volatile component, measured along the channels along which the latter migrates, a the difference between the effective cross-sections of a molecule of the volatile component and of a channel, and x a constant not greatly different from unity.

The more usual case where the volatile component occupies several different sets of lattice positions is also considered, also the variation in ε - η and in a with x. It is emphasized that the equation can only be regarded as a first approximation, but it represents the available data reasonably well.

The approximate ‘condensation areas’, a, found for several ‘zeolitic’ systems are discussed and shown to be in reasonable agreement with what is known of the size of the water-bearing channels in zeolites and of the dimensions of the volatile molecules concerned.

The condition of the water in the zeolites is discussed in the light of the evidence now available.

Kinetic treatment of the rate of diffusion of the volatile component, on the assumption that it only occupies one set of lattice positions, leads to an expression for the diffusion constant: K=[D0{1+ξ(θ)}2Nμ00{1+ζ(x,θ)}∕πXM]×e−Nμ00{1+ζ(x,θ)}∕RT·[1+x(1−x)Nμ00∂ζ(x,θ)∕RT∂x], where θ defines direction in the crystal, D0 the distance between lattice positions for the volatile component in the direction θ = 0, and D0{1 + ξ(θ)} the distance in the direction θ, μ00 the activation energy of migration in ergs per molecule of the volatile component for x = 0 and θ = 0, and μ00 {1 + ξ (x, θ)} the energy for x, θ; M is the molecular weight of the volatile component. The other symbols have the meaning defined above.

The case where the volatile component occupies more than one set of lattice positions is considered, and it is shown that on certain assumptions the experimental data of A. Tiselius are reasonably well reproduced.

Mineralogical Magazine; September 1935 v. 24; no. 150; p. 99-130; DOI: 10.1180/minmag.1935.24.150.01
© 1953, The Mineralogical Society
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